Frank Willis

Observation of transitions to spin‐slip structures and splitting of the Néel temperature of holmium in magnetic fields

We present the results of magnetization measurements on single‐crystal holmium using a SQUID magnetometer in the temperature range from 4 to 140 K in magnetic fields up to 5.5 T. In low fields (0.01 T) the magnetization versus temperature data show a spiral to conical transition at Tc=16 K and the Neel temperature at 132 K. In addition, we observe new anomalies in the temperature dependence of the magnetization along the a, b, and c axes at 20, 24, 42, and 98 K. These new anomalies appear at the same temperatures as observed by Bates et al. [J. Phys. C 21, 4125 (1988); 21, 4113 (1988)] in ultrasonic velocity measurements on holmium. These anomalies could be accounted for within the frame work of the ‘‘spin‐slip’’ model of Gibbs and co‐workers. In the c axis magnetization we observe a splitting of the Neel temperature in magnetic fields greater than 0.5 T. The H‐T phase diagrams of the magnetic phases of holmium for fields in three directions (along the a, b, and c axes) are presented.

Electrodeposition of YBaCuO and ErBaCuO superconductor precursor films

Abstract Precursor thin films of YBaCuO and ErBaCuO superconductor were produced by pulse plating from the dissolved nitrate salts of yttrium, barium, copper and erbium, barium, copper in dimethylformamide. The films were produced on zirconium as well as silvercoated SrTiO3 and CaTiO3. The electrodeposited films were heat treated in flowing O2. Film composition was determined by energy-dispersive X-ray analysis. Critical temperature transitions were determined by magnetic as well as resistivity measurements. Onset critical temperatures were measured up to 93 and 80 K for YBaCuO and ErBaCuO respectively. In resistivity measurements of YBaCuO on CaTiOa, a small step drop at approximately 240 K was observed. This drop in resistivity disappeared after one thermal cycle. Similar drops in resistivity above 200 K have been observed by other authors. Compared with potentiostatic and galvanostatic techniques, the use of pulse plating enhances nucleation and growth and results in uniform and homogeneous films.

Magnetization and thermal expansion of single‐crystal Er and Tm

The magnetization and ac susceptibility data of single‐crystal Er show the antiferromagnetic ordering of the c axis and basal plane moment components at TN∥ = 87 K and TN⊥ = 53 K, respectively. In addition, we observe anomalies at 51, 40, 34, 29, and 27 K. These anomalies correspond to the commensurate spin‐slip structures observed by Gibbs et al. [Phys. Rev. B 34, 8182 (1986)] in x‐ray synchrotron scattering studies. From the thermal expansion study of single‐crystal Er, we find that the phase transitions at TN∥ and 51 K are second order, whereas the transitions at TN⊥, 27, and 18 K are first order. The order of these transitions are in poor agreement with earlier studies of single‐crystal Er. The magnetization measurements of single‐crystal Tm show anomalies at TN = 57 K, Tc = 30 K, and an anomaly at 41.5 K.

Effect of spin slip structures on the resistivity of erbium and holmium

Abstract Single-crystal erbium and holmium each have a series of magnetic phases which have become known as ‘spin slip’ structures. These structures were first observed using X-ray and neutron diffraction by Gibbs et al . Recently the present authors reported anomalies in the temperature dependence of the magnetization of single crystals of holmium and erbium. These anomalies correspond well with the results of Gibbs et al . In this paper the results of resistivity as a function of temperature for pure single crystals of erbium and holmium are reported. In single-crystal erbium, transitions in resistivity are seen at T = 19, 25, 30, 33, 40, 50, 53 and 86 K. Resistivity data for holmium clearly show transitions at T c = 13, 20, 24 and 42 K and T N = 132 K. In addition, a large increase in the electrical noise for holmium in the temperature range 80–105 K is observed. These electrical measurements are the first reported for erbium and holmium to show the effects of spin slip structures on resistivity.

Magnetic properties of the icosahedral phase of Ti transition metal alloys

We report the magnetic properties of the icosahedral phase (i phase) in rapidly quenched Ti‐Mn based alloys. Ribbons of i‐phase samples of composition Ti61Mn37Si2, Ti61Mn22Cr15Si2, Ti61Mn22Fe15Si2, and Ti61Mn22Co15Si2 were prepared by melt spinning onto a copper disk in an argon atmosphere. Low‐field magnetization data reveal a ferromagnetic ordering at Tc ≂ 40 K for Ti61Mn37Si2 samples. A partial substitution of Mn by Cr enhances Tc to 58 K with large thermal hysteresis, whereas samples of composition Ti61Mn22Fe15Si2 show no long‐range magnetic ordering down to 4 K. We have found that the sample Ti61Mn22Co15Si2 shows evidence of spin‐glass behavior below 15 K.

Investigation of the magnetic-non-magnetic crossover region in the Kondo lattice system CeSix

Measurements of resistivity, susceptibility and magnetization for polycrystalline samples of the alloys CeSix (1.6 or=1.90 do not. This correlates well with previous studies of these samples. However, samples with x=1.70 and 1.75 show the presence of an anomalous second peak at a slightly higher temperature than the ferromagnetic ordering temperature. CeSix with x=1.90 shows interesting resistive behaviour with characteristics of both magnetic and non-magnetic samples. It exhibits a broad low temperature peak, a Kondo minimum at higher temperatures and the onset of Fermi liquid behaviour below the maximum. The magnitude of the room temperature resistivity is substantially larger than that of the other members of the series, indicating that this sample is very close to the critical concentration for the magnetic-non-magnetic transition in this series. High temperature anomalies are seen in the resistivities of samples with x<or=1.90; nonetheless, no magnetic correlation is observed. Negative magnetoresistance is observed for samples CeSi1.70 and CeSi1.90 over all fields and temperatures investigated.

Influence of Mn moments on the properties of RMn2 compounds (R=Y and light rare earths)

The influence of Mn moments on the magnetism of RMn2 compounds, where R=Y, Pr, Nd, and Sm, has been investigated using SQUID magnetometry, thermal expansion, electrical resistivity, and magnetoresistance measurements. RMn2 compounds, with R=Y, Pr, Nd, and Sm, order antiferromagnetically at 116, 115, 105, and 94 K, respectively. According to thermal expansion data all these materials undergo a first‐order phase transition from a paramagnetic to a magnetically ordered state, and this effect is believed to be caused by the collapse of the Mn moment at the transition. For the YMn2 compound, the resistivity has a T3 temperature dependence below 38 K and a positive magnetoresistance in the ordered phase, indicating that the Mn moments are localized below TN in this compound. Above TN, however, the Mn moments become itinerant which is reflected as a gradual increase in susceptibility χ with increasing temperature. In PrMn2, NdMn2, and SmMn2 the resistivity does not follow a simple temperature power law and the m...

Magnetization and thermoremanent magnetization of Tb2Mo2O7 and Y2Mo2O7 spin glasses

Abstract Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 are crystalline oxides having the pyrochlore structure. Magnetic susceptibility measurements have shown a magnetic transition which is similar to a spin glass below 27 and 22 K for Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 respectively. Experimental measurements of the low field susceptibility, the magnetic hysteresis loop, the thermoremanent magnetization (TRM) and the time decay of the remanent magnetization have been carried out. A shift from the origin in the magnetic hysteresis loop, the disappearance of TRM at the transition temperature and a log t decay of the remanent magnetization have been observed. All these experimental observations strongly suggest a spin glass behavior for Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 below 27 and 22 K respectively.

Evidence of the c‐axis magnetic moment in single‐crystal Dy

We have studied the magnetic properties of single‐crystal Dy and Tb in the temperature range from 4.2 to 250 K. We observe the Neel transition for Dy and Tb at 180 and 230 K, respectively. The Curie temperature was found to be 90 and 220 K for Dy and Tb, respectively. These values for TN and TC agree well with earlier measurements by Behrendt (1958) and Koehler (1963). In addition to these transitions, we observe a new anomaly in single‐crystal Dy near 4.5 K manifested by a reduction of the magnetization in the basal plane with a simultaneous rise in the magnetization along the c axis. This suggests the magnetic moments may rise out of the basal plane of Dy and form a component along the c axis at low temperatures. We also observe a small broad peak in the ac susceptibility of Dy near 167 K which is likely due to the vortex state of Dy. The ac susceptibility along the b axis of single‐crystal Tb shows a rapid decrease below 30 K while the c‐axis susceptibility shows a corresponding increase.

Magnetism in single‐crystal Dy below 10 K

The resistivity, ac susceptibility, and thermal expansion of single‐crystal Dy have been measured at temperatures below 10 K. Single‐crystal Dy is known to have a ferromagnetic structure with the magnetic moments confined to the basal plane at temperatures below Tc=90 K. At temperatures between Tc=90 K and TN=180 K, single‐crystal Dy has a spiral antiferromagnetic arrangement of the magnetic moments with the screw axis along the c axis. The existence of an additional anomaly near T=6 K in ac susceptibility measurements of a very pure single crystal of Dy has been reported previously. Further evidence for a low‐temperature transition (T<6 K) in single‐crystal Dy is reported. The earlier ac susceptibility anomaly has been reproduced using a second, independently prepared crystal. Thermal‐expansion data show a second‐order phase transition near 5.9 K. A step has been observed intermittently in the resistivity along the a and c axes at 6.5 and 6 K, respectively.